1. Field of the Invention
The present invention relates to a correction table creation method for head position control for a disk device which controls the position of a head to a rotating disk for one or both of reading and writing information from/to the disk, and head position control method and the disk device thereof, and more particularly to a correction table creation method for head position control for correcting the rotation synchronization component of the position signals, and the head position control method and disk device thereof.
2. Description of the Related Art
Disk storage devices for recording to and regenerating from a rotating disk medium are widely used as the storages of data and other information. A disk device is comprised of a disk for storing data, a spindle motor for rotating the disk, a head for recording/regenerating information to/from the disk, and an actuator for moving the head to a target position. Typical examples are magnetic disk devices (HDD: hard disk drive) and magneto-optical disk devices (DVD-ROM, MO).
In a magnetic disk device, a plurality of position signals for detecting the position of the head are recorded in an arc with respect to the rotation center and form a track. A position signal is comprised of a servo mark, track number (gray code) and offset information. The current position of the head can be known by the track number and offset information.
The difference between this position information and target position is determined, calculation is performed according to the displacement amount, and drive signals for driving the actuator, such as current for a VCM (Voice Coil Motor) and voltage for a piezo actuator, are supplied.
The position signal (servo signal) on the disk is either recorded by the disk device itself with use of the STW (Servo Track Writing) method, or recorded by an external STW device. The STW method for recording the position signals by the disk device itself includes pushpin STW, self-servo writing and rewrite STW. The method for recording by an external STW device includes a method for recording on a single disk, magnetic transfer and discrete medium.
In order to accurately record and regenerate data, it is necessary to accurately position the head to the position demodulated from the position signals. But the position signals include noise, which deteriorates the positioning accuracy. This noise has a component synchronizing with the rotation of the spindle motor, and a component not synchronizing with the rotation. The component synchronizing with the rotation can be measured and corrected, and can be suppressed to zero if the measurement is allowed. For the component not synchronizing with rotation, on the other hand, measurement and correction are difficult. Various methods have been proposed to measure and correct the component synchronizing the rotation.
There are two causes that generate the component synchronizing with the rotation of the spindle motor in the position signals in a status where vibration is not received from the outside. The first cause is that the servo signals are not accurately recorded concentrically during STW. As long as the servo signals are mechanically recorded, mechanical, electrical and magnetic noises during recording are unavoidable. Therefore accurately recording the servo signals concentrically during STW is extremely difficult.
The second cause is that distortion occurs to the disk and spindle motor after STW. Microscopically, the position signals are not aligned concentrically with respect to the center of rotation of the spindle motor. The track width of current disk devices is around 200 nano meter, so the influence even of a slight distortion on positioning accuracy is extremely large.
The component of fluctuation of the position signals that synchronize the rotation is called “eccentricity” or “RRO (Repeatable Run Out)”, and the component of a one time rotation frequency is called “primary”, and two times thereof is called “secondary”. If RRO occurs, the positioning accuracy of the head deteriorates, which causes problems in recording and regenerating data. For example, if the positioning accuracy is poor, when data is recorded on a track, a part of previously recorded data on the adjacent tracks is overwritten. To prevent such a status, RRO must be controlled in positioning control.
Conventionally available methods of controlling RRO are a method of controlling the actuator not to follow-up RRO but to ignore it, as shown in FIG. 42, and a control method of controlling the actuator to follow-up RRO, as shown in FIG. 43.
FIG. 42 and FIG. 43 show block diagrams of the head position control system, where as a disturbance is applied to the control system, the components synchronizing the rotation of the spindle motor and the components not synchronizing the rotation thereof are indicated as RRO, NRRO, RPE and NRPE. RRO (Repeatable Run Out) is a component of positional disturbance synchronizing rotation. NRRO (Non-Repeatable Run Out) is a component of positional disturbance not synchronizing rotation. RPE (Repeatable Position Error) is a component synchronizing rotation included in the position error ‘e’ when the positioning control is performed. And NRPE (Non-Repeatable Position Error) is a component not synchronizing rotation included in the position error ‘e’ when the positioning control is performed. The RRO and RPE, which are components synchronizing rotation, indicate different values depending on the sample of the servo.
To perform positioning control, it is necessary to know RRO, RPE and RPE+RRO, and to minimize all these three values. RRO indicates the distortion of the track on the disk. Here the difference from RRO of the adjacent track is critical. This difference value indicates the fluctuation of the track width. As the fluctuation width of the difference values becomes wider, the fluctuation of the track space increases, in other words, areas where the track width is narrower are generated. When vibration is applied from the outside and the positioning accuracy deteriorates, this RRO determines the width where an over-write on the recording area of the data of the adjacent track occurs. In other words, suppressing RRO contributes to improving resistance to external vibration.
RPE indicates the displacement from the positioning target. If control is being performed only to follow-up RRO, RPE indicates a deviation from RRO. In ordinary positioning control, RPE is the management target. For example, if RPE is contained in a plus/minus 15% range from the track width, or if this status continues for a predetermined number of samples, data recording is enabled, or it is judged whether seek control completed (stabilization completed). In other words, suppressing RPE contributes to improving the seek response time.
RPE+RRO indicates a locus of an actuator in a status where no vibration is applied from the outside. That is, this indicates a data recording position when vibration from the outside is zero. As this shows, suppressing RPE+RRO contributes to improving the positioning accuracy and error rate of recording/regenerating data in ordinary operating status where external vibration is low.
In FIG. 42 and FIG. 43 the actual position ‘y’ and the position error ‘e’ can be expressed by the following relational expression (1), using the target position ‘r’, RRO, NRRO, the transfer function of the control system C (z), and the transfer function of the plant (actuator in the case of a magnetic disk device) P (z).
                              y          =                                                                                          C                    ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                        1                  +                                                            C                      ⁡                                              (                        z                        )                                                              ·                                          P                      ⁡                                              (                        z                        )                                                                                                        ⁢              r                        +                                          1                                  1                  +                                                            C                      ⁡                                              (                        z                        )                                                              ·                                          P                      ⁡                                              (                        z                        )                                                                                                        ⁢              RRO                        +                                          1                                  1                  +                                                            C                      ⁡                                              (                        z                        )                                                              ·                                          P                      ⁡                                              (                        z                        )                                                                                                        ⁢              NRRO                                      ⁢                                  ⁢                  e          =                      y            -            r                                              (        1        )            
Now the status during track follow-up, when the target position ‘r’ is always the same and no vibration is applied from the outside, will be considered. The term of the target position ‘r’ at the right hand side in the expression for ‘y’, shown in expression (1), is a constant value, and is equal to the input (target position) ‘r’. Therefore expression (2) of the position error ‘e’ in the track follow-up status is obtained.
                    e        =                                            1                              1                +                                                      C                    ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                            ⁢            RRO                    +                                    1                              1                +                                                      C                    ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                            ⁢            NRRO                                              (        2        )            
Note that ‘e’ in this status is also the sum of RPE and NRPE. Therefore RRO and NRRO and RPE and NRPE are expressed in the following relational expressions (3). In these expressions, 1/(1+C(z)·P(z)) is generally called the “sensitivity function”.
                              RPE          =                                    1                              1                +                                                      C                    ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                            ⁢            RRO                          ⁢                                  ⁢                  NRPE          =                                    1                              1                +                                                      C                    ⁡                                          (                      z                      )                                                        ·                                      P                    ⁡                                          (                      z                      )                                                                                            ⁢            NRRO                                              (        3        )            
In this way, there is a characteristic difference for the amount of the sensitivity function between RPE and RRO. In other words, there is a difference depending on the frequency. This means that even if RRO can be suppressed, the same suppression rate cannot always be implemented for RPE. And even if RRO can be suppressed 50%, RPE may be suppressed only 20%. The difference in frequency characteristics must be considered.
There are two types of methods to suppress RRO, that is, a method for the actuator not to follow-up RRO, as shown in FIG. 42, and a method for the actuator to follow-up RRO, as shown in FIG. 43. Both of these methods have actually been applied to disk devices. A method of using both of these methods is also in use. For example, the actuator follows up the low frequency area and does not follow-up in the high frequency area.
FIG. 42 shows the method for the actuator not to follow up RRO. A correction table 100 for storing values, called a “RroTable” is created in advance. And when the positioning control of the disk device is performed, the RroTable of the correction table 100 is subtracted from the position error ‘e’, and the result is used for the controller C(z). In other words, the controller C(z) calculates the control amount from the position error from which RRO is removed. These RroTable values may be different depending on the head, track, read position or write position, or may be the same for each head or each zone formed by a plurality of tracks.
One of the RroTable generation methods that has been proposed is a method of determining RRO by computing the observing position. An example of this computation method is using a discrete Fourier transform (DFT) (e.g. Japanese Patent Application Laid-Open No. H 11-126444). As FIG. 42 shows, the positional locus, that is RroTable of the correction table 100, is generated from the position error ‘e’, and RPE included in ‘e’ is removed. The position error ‘e’ at this time is expressed by the following expression (4). According to this expression (4), the actuator operates not to follow-up RRO.
                                                        e              =                                                1                                      1                    +                                                                  C                        ⁡                                                  (                          z                          )                                                                    ·                                              P                        ⁡                                                  (                          z                          )                                                                                                                    ⁢                                  (                                      RRO                    +                    NRRO                    -                    RroTable                                    )                                                                                                        =                              RPE                +                NRPE                -                                                      1                                          1                      +                                                                        C                          ⁡                                                      (                            z                            )                                                                          ·                                                  P                          ⁡                                                      (                            z                            )                                                                                                                                ⁢                  RroTable                                                                                        (        4        )            
Therefore to remove RPE from the position error ‘e’, a RroTable is generated so as to satisfy the following expression (5).RroTable=(1+C(z)·P(z))RPE   (5)
In other words, as FIG. 42 shows, RPE is acquired from the position error ‘e’ by the acquisition block 110, and the RroTable value is determined from the acquired RPE through the reverse characteristics of the sensitivity function by the RRO calculation block 112. To determine average values of a plurality of cycles of the disk, the addition block 114 adds the RroTable value of each cycle of the disk.
In the case of the method for the actuator to follow-up RRO, on the other hand, a correction table 118 for storing a value of URroTable is created in advance. And during the positioning control of the disk device, the URroTable value of the correction table 118 is added when a drive signal is supplied from the control system C(z) to the plant P(z). This URroTable value as well may be different depending on the head, track and read position/write position, or may have a value for each head and for each zone which is comprised of a plurality of tracks. Each head may have a URroTable value regardless the track.
For this URroTable generation method as well, some methods have been proposed, such as a method of computing the position error ‘e’ and determining a signal that follows up RRO. Examples of this computation method are a method using repeat control and a method of using a discrete Fourier transform (e.g. see Japanese Patent Application Laid-Open No. H 11-126444).
As FIG. 43 shows, the URroTable value is generated from the position error ‘e’, and RPE included in ‘e’ is removed. As mentioned above, the actuator operates so as to follow-up RRO and the position error ‘e’ satisfies the following relational expression (6) in a track following status where external vibration is not applied.
                                                        e              =                                                                    1                                          1                      +                                                                        C                          ⁡                                                      (                            z                            )                                                                          ·                                                  P                          ⁡                                                      (                            z                            )                                                                                                                                ⁢                                      (                                          RRO                      +                      NRRO                                        )                                                  +                                                                            P                      ⁡                                              (                        z                        )                                                                                    1                      +                                                                        C                          ⁡                                                      (                            z                            )                                                                          ·                                                  P                          ⁡                                                      (                            z                            )                                                                                                                                ⁢                  URroTable                                                                                                        =                              RPE                +                NRPE                +                                                                            P                      ⁡                                              (                        z                        )                                                                                    1                      +                                                                        C                          ⁡                                                      (                            z                            )                                                                          ·                                                  P                          ⁡                                                      (                            z                            )                                                                                                                                ⁢                  URroTable                                                                                        (        6        )            
Therefore in order to remove RPE from the position error ‘e’, a URroTable value is generated so as to satisfy the following expression (7). Expression (7) indicates that the URroTable value is determined from RPE through the transfer function that has a reverse characteristic of the sensitivity function and the reverse characteristic of the plant.
                    URroTable        =                              -                                          1                +                                  C                  ⁢                                                            (                      z                      )                                        ·                                          P                      ⁡                                              (                        z                        )                                                                                                                        P                ⁡                                  (                  z                  )                                                              ⁢          RPE                                    (        7        )            
In other words, as FIG. 43 shows, RPE is acquired from the position error ‘e’ by the acquisition block 110, and the URroTable value is determined from the acquired RPE through the reverse characteristic of the sensitivity function and the reverse characteristic of the plant by the RRO calculation block 112. If an average value of a plurality of cycles of the disk is determined, the URroTable value of each cycle of the disk is added by the addition block 114.
As described above, a method of determining the correction table RroTable or URroTable from the observed RPE is to convert through such a transfer function as (1+C(z)·P(z)) or−(1+C(z)·P(z))/P(z). For this conversion method, a method of determining the frequency characteristic between RPE and Rro or URro in advance and using a discrete Fourier transform (DFT) is best to determine waveforms most accurately.
A problem of generating the waveform of RroTable and URroTable is noise, particularly the asynchronous component in the position disturbance: NRRO. The NRRO is observed as NRPE during positioning control. A waveform is determined using position signals regardless whether the actuator follows up the RRO or not, but the problem occurs when NRPE is most obviously large compared with RPE.
When the NRPE is “0”, RPE can be determined simply by observing one cycle of position signals. Therefore it is easy to generate a RroTable or URroTable for the actuator to follow-up RRO completely, or not to follow-up with RRO completely. However in reality the magnitude of RPE and NRPE are about the same, and a method of considering the error of NRPE during RPE measurement is required.
A method of decreasing the influence of NRPE is a method of averaging the position error ‘e’. Position signals are measured continuously for a plurality of times of rotation cycles (e.g. 100 cycles), and an average value of the measured values is determined for each servo sector. This average value is regarded as the RPE, and a correction signal for not following up with RRO or for following up with RRO: a Rro or URroTable value, is calculated. This Rro or URroTable value is calculated assuming that no noise is included in the observed waveforms. And this value is substituted for each servo sector in the correction tables 100 and 108 of the RroTable and URroTable.
A method of simply averaging can decrease the influence of NRPE, but cannot suppress it to “0”. Also the standards of magnitude of NRPE with respect to RPE and the number of rotation cycles to be measured are vague. So it has been proposed to multiply the averaged position error signals by a gain smaller than 1: Krro or Kurro, and substitute the result in table 100 or 118 (e.g. U.S. Pat. No. 6,437,936 B1).
According to this method, when NRPE is large, or when the number of rotation cycles for measurement is small, Krro or Kurro is adjusted by experiment so that RPE after correction becomes small. The gain becomes closer to “0” when NRPE is large with respect to RPE, and becomes closer to “1” if small. To solve this uncertainty of gain, the measurement and correction of RPE are further repeated (e.g. see U.S. Pat. No. 6,437,936, B1). The case when the measurement and correction are repeated twice for example, will be considered. By using Rro [1] or URro [1] determined by the first measurement and calculation, a correction table is generated using the following expression (8).RroTable[1]=Krro[1]·Rro[1]URro Table[1]=Kurro[1]·URro[1]  (8)
RPE is measured a second time in the status after positioning control is performed using the generated correction table RroTable [1] or URroTable [1]. Then Rro [2] or URro [2] is calculated again and added to the previous correction table according to the following expression (9).RroTable[2]=Krro[2]·Rro[2]+RroTable[1]URroTable[2]=Kurro[2]·URro[2]+URroTable [1]  (9)
Today track density is increasing along with demands for increased storage capacity. FIG. 44 shows the fluctuation of three adjacent tracks, where the locus of the track center differs depending on the track. FIG. 45 shows the locus of the top and bottom tracks viewed from the center track in FIG. 44, and indicates the track width of the center track. In this way, a highly accurate RRO correction is demanded because of the high density of the tracks.
In the conventional method of generating an RRO correction table: RroTable or URroTable of a disk device, a method to theoretically determine an optimum value of the gain: Krro or Kurro, when Rro and URro, that is a correction value, is added to the correction table, has not been established. Therefore it is unavoidable that an optimum value must be determined by experiment based on experience.
Therefore the suppression rate of RPE after correction cannot be estimated and must be confirmed by experiment. This is a problem when the suppression rate is adjusted for each disk device, making it impossible to estimate the positioning accuracy value when a new device is designed.
Also when the correction value is measured in the manufacturing steps of the disk device, it is difficult to set a standard for the RPE measurement time, that is the manufacturing time, required for satisfying the specifications of the positioning accuracy. For example, according to a prior art, a more accurate RRO correction table can be created as the measurement time becomes longer (e.g. several hours), to satisfy the specifications of the positioning accuracy, but as the measurement time becomes longer, the ratio of the correction table creation time in the manufacturing steps becomes longer, which is inappropriate for manufacturing large quantities of disk devices.
Also when gain is determined by experiment, the frequency characteristic of the position error is not considered, so it is difficult to effectively suppress RPE after correction, which may drop the follow-up accuracy of the disk device.